Intrinsically Re-curable Photopolymers Containing Dynamic Thiol-Michael Bonds

The development of photopolymers that can be depolymerized and subsequently re-cured using the same light stimulus presents a significant technical challenge. A bio-sourced terpenoid structure, l-carvone, inspired the creation of a re-curable photopolymer in which the orthogonal reactivity of an irreversible thioether and a dynamic thiol-Michael bond enables both photopolymerization and thermally driven depolymerization of mechanically robust polymer networks. The di-alkene containing l-carvone was partially reacted with a multi-arm thiol to generate a non-crosslinked telechelic photopolymer. Upon further UV exposure, the photopolymer crosslinked into a mechanically robust network featuring reversible Michael bonds at junction points that could be activated to revert, or depolymerize, the network into a viscous telechelic photopolymer. The regenerated photopolymer displayed intrinsic re-curability over two recycles while maintaining the desirable thermomechanical properties of a conventional network: insolubility, resistance to stress relaxation, and structural integrity up to 170 °C. Our findings present an on-demand, re-curable photopolymer platform based on a sustainable feedstock.


General Materials and Methods
All compounds, unless otherwise indicated, were purchased from commercial sources, and used as received. Methyl 3-mercaptopropionate was distilled and stored under a nitrogen atmosphere in an ampoule. NMR Spectroscopic Analysis. All NMR spectroscopy experiments were performed at 300 K on a Bruker DPX-400 NMR instrument equipped with a BBFO smart probe operating at 400 MHz for 1 H (100.57 MHz for 13 C). 1 H NMR spectra are referenced to solvent residual proton (δ = 7.26 for CDCl3, δ = 2.91 for DMF-d7) and 13 C NMR spectra are referenced to the solvent signal (δ = 77.16 for CDCl3, δ = 162.7 for DMF-d7).
Thermogravimetric Analysis (TGA). TGA thermograms were obtained using a Q550 Thermogravimetric Analyzer (TA instruments). Thermograms were recorded under an N2 atmosphere at a heating rate of 10 °C‧min -1 , from 10 to 600 °C, with an average sample weight of ca. 5 mg. Aluminium pans were used for all samples. Decomposition temperatures were reported as the 5% weight loss temperature (Td,5%).
Fourier-transform infrared (FTIR) spectroscopy. FTIR spectra were collected out using an Agilent Technologies Cary 630 FTIR spectrometer. 16 Scans from 600 to 4000 cm -1 were taken at a resolution of Rheology. Resin viscosity values were obtained from shear rate sweeps from 0.1 to 100 s -1 on an Anton Paar MCR 302 using with a PP25 geometry. Resins were loaded onto the plate at ambient temperature with a gap of 0.5 mm.
Stress-Relaxation was performed on an Anton Paar MCR-302 using Anton Paar PP8 parallel-plate, with a diameter of 8 mm. Temperature was controlled with a P-PTD 200/AIR Peltier and a P-PTD 200 hood.
Strain sweeps were performed to ensure stress relaxation experiments were performed in the linear region.
Stress relaxation tests were performed at 2% strain at 100 °C and 140 °C. (n = 1) Photorheology. The crosslinking kinetic of the resins was examined as a function of gelation time by photorheology using an Anton Paar MCR-302 rheometer fitted with a detachable photoillumination system (Exfo OmniCure S1500 UV light source, broadband Hg-lamp, glass plate). Resin samples were sheared between two parallel plates at 0.2 Hz with an amplitude of 25% for 50 s without irradiation. After this time, the light source was switched on and measurements were taken every 0.2 s over the course of 450 s. The intersection point of the storage moduli and loss moduli plots was used to determine the time of gelation of the resin.
Dynamic Mechanical Analysis (DMA). Dynamic mechanical thermal analysis (DMTA) data were obtained using a Mettler Toledo DMA 1 star system and analyzed using the software package STARe V13.00a Thermal sweeps were conducted using films (L x W x thickness = 15 mm x 6 mm x 0.5 mm) cooled to −50 °C and held isothermally for ca. 5 minutes. Storage and loss moduli, as well as the loss factor (ratio of E" and E', tan δ) were probed as the temperature was swept from −50 to 180 °C, 5 °C•min −1 .
Thermomechanical behavior was determined from three samples in this way. (n = 3, unless otherwise specified) Tensile testing. Uniaxial tensile testing was performed using a Testometric M350-5CT universal mechanical testing instrument fitted with a load cell of 10 kN. Film samples were visually inspected for deformation and bubbles then dumbbell shaped samples were cut using custom ASTM Die D-638 Type 5.
Each specimen was clamped into the tensile holders and subjected to an elongation rate of 10 mm·min -1 until failure. All tensile tests were repeated 3 times, and an average of the data was taken to find the ultimate tensile stress and strain. Data was analyzed using winTest™ Analysis software (v.5.0.34) and OriginPro® software.
Swell testing and gel fraction. Networks were taken and cut into disks (6.7 × 0.5 mm) and weighed before being submerged in THF and allowed to swell until equilibrium swelling was reached after 24 hours. The initial mass (Wi) and swelling mass (Ws) was used to calculate the swelling ratio (%) as shown in equation S1. The swollen networks were resubmerged in new THF and finally dried to get the dry mass (Wd). This was used to calculate the gel fraction (%) as shown in equation S2.

Synthesis of CarvMMP
L-carvone (2.00 g, 1 equiv, 13.3 mmol) and methyl 3-mercaptopropionate (1.76 g, 1.1 equiv, 14.7 mmol) were mixed in bulk at 0 °C until homogenous. DBU (19.5 μL, 1 mol%) was then added in one-portion and the reaction was left to stir at 0 °C for 30 min before warming to ambient temperature. The mixture was purified using silica column chromatography with EtOAc/Hexanes and concentrated yielding a colorless oil (1.9 g, 50%

Radical addition of MMP to carvone model reaction
L-carvone (0.8513 g, 1 equiv, 5.68 mmol) and methyl 3-mercaptopropionate (1.4303 g, 2.1 equiv, 11.9 mmol) were mixed in bulk until visibly homogenous in a glass vial. TPO-L (0.022 g, 1 wt%) was then added in one portion and vigorously stirred. The mixture was then irradiated with UV light in a Formlabs Form Cure 405 nm curer and sampled periodically (10 s, 30 s, 60 s, 5 min, 10 min, 30 min, 1 h, 1.5 h, 2 h, 3 h) for 1 H-NMR spectroscopy. Data was plotted against irradiation time and lines of best fit were generated in OriginPro 2019 (9.6.0.172) using the exponential decay fitting function.

Dynamic exchange model reaction ambient temperature
CarvMMP (135 mg, 0.5 mmol) was dissolved in DMF-d7 (2 mL) in a glass vial, then DBU (3.7 µL, 5 mol%) was added and stirred rapidly. An aliquot was taken for an initial 1 H NMR spectrum. The remaining solution was left stirring at ambient conditions and sampled periodically (30 min, 1 h, 2 h) for 1 H NMR spectroscopy.

Depolymerization of 1-CarvFilm to produce 2-CarvPrepolymer
Shredded 1-CarvFilm (10.55 g, 1 equiv, 16.9 mmol), DMF (67 mL) and DBU (126 μL, 5 mol%) were added to an ampoule with a magnetic stirrer and stirred at ambient temperature. Nitrogen is bubbled through the mixture for ~ 30 min before the flask is sealed and heated at 140 °C for ca. 16 h. The reaction is removed from the heat and filtered through fiberglass wool to remove the insoluble particulates, the residue is dried and weighed to assess the quantity of particulates (49 mg, 0.5% of the original weight). The filtrate is precipitated into methanol (~800 mL), centrifuged and the supernatant is discarded. The remaining orange tacky solid is washed with additional methanol (~200 mL) and then the supernatant was discarded. Residual

Depolymerization of 2-CarvFilm to produce 3-CarvPrepolymer
Shredded 2-CarvFilm (3.28 g, 1 equiv, 5.26 mmol), DMF (21 mL) and DBU (39 μL, 5 mol%) were added to an ampoule with a magnetic stirrer and stirred at ambient temperature. Nitrogen is bubbled through the mixture for ~ 30 min before the flask is sealed and heated at 140 °C for ca 16 h. The reaction is removed from the heat and filtered through fiberglass wool to remove the insoluble particulates, the residue is dried and weighed to assess the quantity of particulates (3.7 mg, 0.1% of the original weight). The filtrate is precipitated into methanol (~350 mL), centrifuged and the supernatant is discarded. The remaining orange tacky solid is washed with additional methanol (~100 mL) and then the supernatant was discarded. Residual

Depolymerization of 1-CarvFilm and precipitation into distilled water
Shredded 1-CarvFilm (0.6508 g, 1 equiv, 1.0 mmol), DMF (4.2 mL) and DBU (7.7 μL, 5 mol%) were added to an ampoule with a magnetic stirrer and stirred at ambient temperature. Nitrogen is bubbled through the mixture for ~ 30 min before the flask is sealed and heated at 140 °C for ca. 16 h. The reaction is removed from the heat and filtered through fiberglass wool to remove the insoluble particulates. The filtrate is precipitated into water (~80 mL), centrifuged and the supernatant is discarded. The remaining mixture is freeze-dried to remove residual water to yield a tacky orange solid 2-CarvPrepolymer (0.4653 g, 71%).

Attempted depolymerization of 1-LimFilm
Shredded 1-LimFilm (0.7377 g, 1 equiv, 1.2 mmol), DMF (4.9 mL) and DBU (9 μL, 5 mol%) were added to an ampoule with a magnetic stirrer and stirred at ambient temperature. Nitrogen is bubbled through the mixture for ~ 30 min before the flask is sealed and heated at 140 °C for ca. 16 h. The reaction is removed from the heat and filtered through fiberglass wool to remove the unreacted 1-LimFilm, the residual material is dried and weighed to assess the mass loss 0.7298 g, 99% of the original weight (or 1% weight loss).

Formulation of 1-CarvResin
1-CarvPrepolymer (7 g, 70 wt ratio) was added to a vial and stirred in dimethylcarbonate (3 g, 30 wt ratio) until the liquid was homogenous. The vial was wrapped in foil and TPO-L (0.14 g, 2 wt%) was added, then the vial was agitated using an orbital shaker for 1 h until the liquid was homogenous.

Formulation of 2-CarvResin
2-CarvPrepolymer (4.94 g, 55 wt ratio) was added to a vial and stirred in dimethylcarbonate (4.04 g, 45 wt ratio) until the liquid was homogenous. The vial was wrapped in foil and TPO-L (0.0988 g, 2 wt%) was added, then the vial was agitated using an orbital shaker for 1 h until the liquid was homogenous.

Formulation of 3-CarvResin
3-CarvPrepolymer (1.87 g, 55 wt ratio) was added to a vial and stirred in dimethylcarbonate (1.53 g, 45 wt%) until the liquid was homogenous. The vial was wrapped in foil and TPO-L (0.0374 g, 2 wt%) was added, then the vial was agitated using an orbital shaker for 1 h until the liquid was homogenous.

General resin curing procedure
Resins were deposited on a glass slide using a glass pipette to ensure a flat surface and even coverage.
The glass slide is positioned above the UV light source (an OmniCure S1500 fitted with a fiber optic cable) and irradiated from beneath the glass slide at approx. 15 cm. Note: positioning the light source below the glass slide ensured thick films did not exhibit surface wrinkling and defects over extended curing periods.
The resins were inspected periodically until the surface of the film was solid and non-tacky (typically ~1 hour for 1 mm thickness). The films were removed from the glass slide and post-cured in a vacuum oven at 90 °C under reduced pressure for ca. 16 h to remove dimethyl carbonate and ensure the network was sufficiently crosslinked. Figure S1. UV-Vis spectrum of TPO-L in NMP with vertical line illustrating absorption at 405 nm. Figure S2. 1 H NMR spectra of the addition of MMP to L-carvone via radical mediated thiol-ene at increasing radiation times in CDCl3. Consumption and formation of each functionality was referenced against the methyl ester singlet at 3.69 ppm and summarized in Table S1. Table S1. Molar percentage of the monitored functionalities in the radical addition of MMP to L-carvone against UV irradiation time. Dynamic exchange experiment at ambient temperature Figure S4. The dynamic exchange of CarvMMP at ambient temperature with and without DBU (5 mol%) at different reaction times. Enone peak at 6.85 ppm was quantified from the unconsumed isopropenyl singlet at 4.80 ppm in DMF-d7.

Van't Hoff plot
Calculation of the Keq for the dissociation of CarvMMP in DMF-d7 with DBU (5 mol%) was achieved from quantifying the carvone concentration at increasing reaction temperatures. Quantifying the L-carvone concentration was achieved through the integration of the characteristic enone singlet at 6.85 ppm in DMF-d7 against the propenyl singlet at 4.80 ppm in DMF-d7 which was unconsumed in the process. Assuming the absence of side reactions during the experiment, the concentration of Carvone and MMP will be equal.

Improved weight recovery through precipitation into distilled water
We hypothesized the material weight recovery was non-quantitative due to the loss of low molecular weight fractions during the precipitation step. Using distilled water (as opposed to methanol) afforded a higher material recovery of 71% (55% for methanol precipitation